(a) Field of the Invention
The present invention relates to a semiconductor Mach-Zehnder modulator and, more particularly, to a semiconductor Mach-Zehnder modulator for use in an optical communication systems or optical data processing systems and driven with a single driver in a push-pull modulation scheme.
(b) Description of the Related Art
Recently, optical communication techniques are directed more and more to high bit-rate transmission. In the optical communication systems, most optical fibers installed in the world, especially around the North America, are 1.3 .mu.m zero dispersion fiber, and provides minimum loss in 1.55 .mu.m range in these fibers. Conventionally, a semiconductor laser direct modulation technique has been generally used in the optical communication. This technique, however, involves wavelength chirping due to dispersion when a high bit-rate 1.55 .mu.m range optical signal is transmitted through a 1.3 .mu.m zero dispersion fibers, thereby causing a signal distortion. The level of the distortion is generally proportional to a (bit rate).sup.2.times.(transmission distance) product.
The problem wavelength chirping can be solved to some extent by employing an external modulation technique. Among other external modulators, an absorption type modulator exhibits smaller chirping compared to semiconductor lasers; however, not zero. On the other hand, if a Mach-Zehnder modulator, which uses optical interference as its operational principle, is used as an external modulator operating in a push-pull modulation scheme, the wavelength chirping can be entirely removed theoretically. Accordingly, Mach-Zehnder modulators are expected to be key external modulators for use in ultra high-speed and long distance optical communication systems.
Some known Mach-Zehnder modulators have dielectric substances such as LiNbO.sub.3. On the other hand, semiconductor Mach-Zehnder modulators are considered to be advantageous over the dielectric type Mach-Zehnder modulators, in view of the integration capability with optical elements such as semiconductor lasers or semiconductor optical amplifiers and electric elements such as FETs, as well as in view of their smaller dimensions and lower power consumption. FIG. 1A shows a conventional semiconductor Mach-Zehnder modulator in a perspective view, and FIG. 1B is a cross-sectional view taken along X--X in FIG. 1A.
The semiconductor Mach-Zehnder modulator of FIG. 1A comprises an input waveguide 6, a pair of input branch waveguides 7-1 and 7-2 branching off input waveguide 6, a pair of phase modulators 8-1 and 8-2 receiving inputs from respective branch waveguides 7-1 and 7-2, a pair of output branch waveguides 9-1 and 9-2 receiving outputs from respective phase modulators 8-1 and 8-2, and an output waveguide 10 receiving combined output from output branch waveguides 9-1 and 9-2.
The Mach-Zehnder modulator of FIG. 1A is fabricated by depositing consecutively undoped InP layer 102, undoped In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y layer 103 (.lambda..sub.PL =1.3 .mu.m), p-type InP layer 104 on an n-type InP substrate 101, patterning specified deposited layers to form a combined mesa structure, and forming independent drive electrodes 105-1 and 105-2 and a common electrode 106 of the phase modulators, as shown in FIG. 1B.
The semiconductor Mach-Zehnder modulator generally uses changes in the refractive index generated upon a reverse-bias voltages applied to a p-n junction. The optical characteristic of the semiconductor Mach-Zehnder modulator is shown in FIG. 2, wherein the optical output thereof is plotted against the drive voltage (reverse bias voltage). The curve denoted by "V1" shows a single arm drive wherein one of the modulators is driven, whereas the curve denoted by "V1 & V2" shows a double arm drive wherein both the modulators are driven for a push-pull modulation.
FIG. 3 shows a timing chart for the push-pull modulation of the modulator such as shown in FIGS. 1A and 1B, wherein modulator 8-1 is applied with a reverse bias voltage V1 through electrode 105-1 changing between 0 and V.sub..pi./2 whereas modulator 8-2 is applied with a reverse bias voltage V2 through electrode 105-2 changing between V.sub..pi./2 and V.sub..pi. in opposite phase with respect to the voltage V1, wherein V.sub..pi. provides a phase shift of .pi. to the phase modulator whereas V.sub..pi./2 provides a phase shift of .pi./2. As shown in FIG. 2, drive voltage for the double arm modulation (V1 & V2), i.e., push-pull modulation, is about a half of that for the single arm modulation (V1) for a specified optical output.
John C. Cartledge et al. report that a double arm modulation scheme achieves a transmission distance which is double the transmission distance obtained by a single arm modulation scheme, in an article "Dispersion Compensation for 10 Gb/s Lightwave Systems Based on a Semiconductor Mach-Zehnder Modulator", IEEE Photonics Technology. Letters, 1995 February, Vol. 7, No. 2, pp 224-226.
FIG. 4 shows full pulse width (ps) at half maximum for a Gauss pulse plotted against fiber length (km) for a single arm modulation and a double arm (push-pull) modulation of the Mach-Zehnder modulator, obtained in our experiments. As understood from FIG. 4, push-pull modulation achieves a small waveform distortion due to pulse compression and thus maintains a half-value width better than a single arm modulation. From the results, it is considered that the push-pull modulation can provide a double or triple transmission distance compared to the single arm modulation.
A push-pull driven semiconductor Mach-Zehnder modulator, such as 301 shown in FIG. 5, generally requires a pair of drivers 200-1 and 200-2 for applying drive voltages to electrodes 302 and 303 of the respective phase modulators and a timing generator 203 for driving the phase modulators 301 exactly in opposite phases. It is difficult to accurately adjust the timing by the timing generator 203, especially at higher frequencies, for example, over 2.5 Gb/s, which fact renders the operation of the phase modulator arms to be difficult at such high frequencies.